US3615723A

US3615723A - Spray-drying apparatus

Info

Publication number: US3615723A
Application number: US28780A
Authority: US
Inventors: Reginald E Meade
Original assignee: Pillsbury Co
Current assignee: Damrow Co Inc; Pillsbury Co
Priority date: 1970-04-15
Filing date: 1970-04-15
Publication date: 1971-10-26
Anticipated expiration: 1988-10-26

Abstract

Process for drying fluid and solid material in which a liquid spray is introduced into a drying chamber together with a source of dry desiccating gas such as heated air. A screen is positioned to receive the spray while the droplets are in a tacky condition to form a mat on the screen with a provision for drawing air from the heated enclosure through the mat and thence through the screen for the purpose of drying the mat. At least portions of the mat are subdivided into a particulate form and recirculated back to the inlet of the apparatus where they are mixed with fresh incoming product.

Description

United States Patent [72] lnventor Reginald E. Meade Stillwater, Minn. [21] Appl. No. 28,780 [22] Filed Apr. 15, 1970 [45] Patented Oct. 26, 1971 [73] Assignee The Pillsbury Company Minneapolis, Minn. Continuation-impart of application Ser. No. 553,101, May 26, 1966.

[ 54] SPRAY-DRYING APPARATUS 15 Claims, 2 Drawing Figs.

[52] US. Cl 99/206, 159/48, 34/11 [51] Int. Cl A23b 7/02 [50] Field of Search 34/15, 11, 9; 159/D1G. 4, 48 R; 99/205, 206, 201, 203; 210/68 [56] References Cited UNITED STATES PATENTS 1,361,238 12/1920 Fleming 99/206 Primary ExaminerFrederick L Matteson Assistant Examiner-Robert A. Dua

Att0rneysRonald E. Lund, James V. Harmon and M. Paul Hendrickson ABSTRACT: Process for drying fluid and solid material in which a liquid spray is introduced into a drying chamber together with a source of dry desiccating gas such as heated air. A screen is positioned to receive the spray while the droplets are in a tacky condition to form a mat on the screen with a provision for drawing air from-the heated enclosure through the mat and thence through the screen for the purpose of drying the mat. At least portions of the mat are subdivided into a particulate form and recirculated back to the inlet of the apparatus where they are mixed with fresh incoming product.

I28 /2 a ,2 P f /a M HEATER osnummr- 'IER. AIR /z2 FILTER I; a 73 E cons-mm an: DRYING 7 PLUS mxms POROUS Ben spa AIR our AIR our AIR our PUMP I is: as no CRJsHmG MATERIAL RECYC E-FEEDBACK or any COOL msmmnou I33 I80 pmmtzs AND AIR TO MIX 3' '7 war PIECES I AND /OR MOIST SPRAY PARYECLES IN OROUS BED SPRAY-DRYING APPARATUS This application is a continuation-in-part of application Ser. No. 553,101, filed May 26, 1966, for Spray Drying Method, Apparatus and Product" by Reginald E. Meade.

The present invention relates to a process for drying liquids and solids and more particularly to a drying process in which a dry substance is mixed with a liquid spray that is to be dried, e.g., a portion of the finished product is subdivided and returned to the inlet end of the apparatus where it is mixed with fresh incoming product.

In the past several decades the drying of materials has been carried out on an ever increasing scale by spraying the liquid that is to be dried into a hot dry stream of gas passing through a drying chamber and collecting the dried solids. This process frequently referred to as spray drying is especially widely used in the food industry for drying such products as milk and cream, instant coffee, cocoa, fruit and vegetable juices, extracts and flavorings. A variety of other products such as drugs, detergents, soaps, cosmetics, etc. have also been dried in this manner. These processes have not been entirely satisfactory. While the process referred to in my prior application noted hereinabove overcomes many of the deficiencies of prior spray drying equipment. In addition to the problems discussed specifically in the above-noted application, there are special problems associated with the drying of specific food products. In drying food, for example, it is possible to proceed in a straightforward manner by spray drying where a puree is desired. n the other hand, where it is necessary to provide the product in pieces or chunks, hot air drying or the like must be resorted to. Hot air drying systems have not been entirely satisfactory because hot air is in contact with each of the pieces being dried only at their surfaces and the drying tends to seal off the pores on the surface thereby producing a skinning effect which tends to inhibit further drying. Moreover, there is no effective mechanism for attracting moisture from the interior of the piece to the exterior. Thus, where hot air drying is used, relatively long exposure time is required even when the air is at a relatively high temperature.

Problems of a different kind are encountered in the drying of solutions which when dried rapidly tend to form a glass rather than a product in a crystalline form. Sucrose is an example of such a product. Thus, when sucrose is sprayed from a nozzle into the heated air stream of a spray drier, the minute droplets dry in such a manner as to cause the sucrose to dry as a glass in which a substantial amount of water is incorporated. Sugar in this form does not meet the standards for most applications in which sugar is used. My prior U.S. Pat. No. 2,627,463 describes a process and apparatus for preparing lactose in which some of the finished dry product is reintroduced to the desiccating chamber. This process was, however, found unsuitable for preparing commercially acceptable sucrose and other relatively deliquescent materials such as amorphous lactose. While it is not known with certainty, this appears to result from a lack of intimate contact of the spray and the recirculated product.

In view of these and other deficiencies of the prior art, it is an object of the invention to provide an improved spray drying process having the following advantages and characteristics: (a) the ability to dry sucrose and other relatively deliquescent materials effectively without the formation of a dry powder that tends to pass through the collection screen or dry as a glass; (b) the capability of efiiciently drying food chunks or slices; (0) suitability for the production of a porous dried mat that can be subdivided into pieces of a predetermined size which if water dispersible will become moistened quickly when added to a liquid; (d) increased drying capacity and better thermal efficiency than prior processes; (e) the ability to produce a dried product of a uniform consistency; (f) a provision for removing the collected mat from contact with the heated drying gas and exposing it to a cooling gas until the dried product reaches a chemically or thermally stable condition.

These and other more detailed and specific objects will be apparent in view of the following specification and attached drawings wherein:

FIG. 1 is a schematic flow diagram illustrating the application of the invention in drying solutions in which a portion of the dried product is subdivided and reintroduced at the inlet of the spray drier in particulate form.

FIG. 2 is a schematic flow diagram of another embodiment of the invention in which the process is applied to the drying of moist food pieces such as fruit slices.

Briefly, the invention provides a process for drying fluid and solid material in which a liquid spray is introduced into a drying chamber together with a source of dry-desiccating gas such as heated air and in which a screen is positioned to receive the spray while the droplets are in a tacky condition to form a mat on the screen with a provision for drawing air from the heated enclosure through the mat and thence through the screen for the purpose of drying the mat. At least portions of the mat are subdivided into a particulate form and recircu lated back to the inlet of the apparatus where they are mixed with fresh incoming product.

The process will now be described in general terms. Starting material can consist of any liquid in which a solid is either dissolved or suspended. Examples are milk, vegetable or fruit juices, sugar solutions, etc. To begin the process, the liquid is sprayed into a drying atmosphere composed for example of air heated to between about 200 and 400 F. or air having a relatively low humidity level. The moisture present in the drying atmosphere is substantially below its saturation point. The term drying atmosphere" used herein is intended to mean a gas at such conditions of temperature, pressure and vapor content as to be capable of absorbing a substantial quantity of liquid from the fluid to be dried.

In accordance with the invention, the liquid to be dried is sprayed into an enclosure containing a drying atmosphere and is collected upon a foraminous element such as a screen. A blower or other draft-producing device draws the drying atmosphere through the accumulated mat that forms on the screen and thence through the screen itself. The pressure differential produced across the screen in this way results in a steady flow of gas through the screen and the accumulating mat. This was found highly effective in assuring product uniformity, improving thermal efliciency and in achieving a low moisture level in the finished product. 4

It is important to have the screen placed with respect to the spray nozzle such that the surfaces of the particles are still in a tacky condition at the time they strike the mat on the screen but do not contain sufficient liquid to enable them to coalesce or run together to form a relatively impermeable continuous material. It was surprising to find that the tacky condition of the particles at the time they reached the screen was efi'ective in preventing an excessive pressure drop across the screen (screen blinding). Thus, by properly positioning the screen with respect to the sprayer (under a given set of drying conditions and spray droplet size), the porosity of the mat is preserved.

By the term coalesce" as used herein I means a flowing or fusing together of the particles to form a relatively solid material in which the individual particles cannot be clearly recognized without magnification and wherein the spaces or interstices between them are relatively small (if present at all) compared with the size of the particles.

The particles are thus brought into contact with one another while the surfaces thereof are in a tacky condition to form bonds at the points of contact between the particles. This produces a relatively porous lacy reticulum in which a substantial portion of the mat thus formed consists of communicating spaces or pores between the contacting and bonded particles.

Once the particles have been deposited on the collection screen, a sudden change will take place in the relation between the particles and the surrounding air; the air will begin to stream over and around the particles and through the minute microporous interstices between the particles in the agglomerate. As this streaming takes place, moisture is removed from the particles at a much higher rate. Removal of the last traces of removable moisture from a material that is being dried takes place much more slowly than the initial drying phase. This terminal-drying phase is commonly referred to as the falling rate drying period. The substantial increase in the velocity of the drying gas relative to the particles established in the present invention (after the movement of the particles has been arrested by deposition on the bed or screen) provides a highly effective means for removing moisture during the falling rate drying period.

As described briefly above, all or a portion of the finished product .is returned to the drying section of the apparatus and at that point is intennixed with the incoming droplets of material falling through the spray drying chamber. To accomplish this objective, the material that is recycled is preferably subdivided to a powdered or granular form. If granular in form, the granules can have a diameter of up to one-sixteenth to one-fourth inches or more. The recycled material in some applications is introduced together with the liquid spray at the top of the drying chamber. In other applications, it is introduced directly onto the drying screen either within the spray drying chamber or, in the event a moving screen is employed, onto a portion of the screen just about to enter the drying chamber.

In summary, the invention has the following advantages: First, filtration of air-entrained particles from the drying medium is accomplished by those particles already deposited in the mat, thus reducing and in some cases eliminating the need for auxiliary filters in the stream of exit drying gas. Since this phenomenon in the course of a drying operation has heretofore been unknown, it will be referred to as autofiltration. Second, greater drying efficiency can be achieved than was heretofore possible. While the precise reason for this result is not known with certainty, it is believed to be, in part, due to the accelerated evaporation that results from the streaming of the drying gas around and through the particles that make up the mat and, in part, to the increased driving force across the solid and gaseous interface. Third, greater product dryness is accomplished with the invention because of the increased exposure of the material to the drying atmosphere. Fourth, the invention is capable of reducing the undesired accumulation of the product on the walls of the drier. This allows the invention to be used with products which were heretofore difficult or impossible to dry by conventional spray drying techniques or, for that matter, by any previous drying process of which I am aware. The invention also increases the product quality because of the reduction of heat damage. Fifth, the product that is being dried is placed, while in a practically dried state, in a form which can be transferred from the drying zone to one or more additional zones in which further treatment may be performed. Sixth, the invention places the product in a novel physical form; a porous, self-supporting mat which can be further processed if desired to provide products having advantageous properties.

Reference will now be made to the important process varia bles. First, any one or several of a variety of substances can be dried including both liquids, e.g., fruit juice, milk or solids such as fruit or vegetable slices. The liquid will ordinarily have a water base but can also have a nonpolar base such as an organic solvent.

The solid substance contained in the liquid can comprise any of a variety of particulated insoluble or soluble materials capable of being dried including both dissolved solids or suspended'material. The term solids" as used herein also includes suspended materials which sometimes exist as a liquid, for example, fat in the case of dairy products.

The material being dried can consist of a single chemically pure substance or a mixture of substances. It must, however, include as a component a nonvolatile material which is rendered tacky or adherent at some point during the removal of liquid. This will, of course, include materials which are not normally considered tacky in the sense that they have finger tack. The term tacky as used herein means capable of forming adhesive connections between particles of sufficient strength to hold a mass of material together to provide a porous medium permitting passage ofgas.

The drying atmosphere will normally consist of heated air having a relatively low moisture content but it can also consist of gases such as nitrogen, carbon dioxide, water vapor or the like. If organic solvents are employed for suspending the solids, an inert atmosphere will be preferred to prevent hazardous operation. As to temperature of the drying atmosphere, any suitable temperature may be used. In general, the drying temperatures normally used in spray drying are used. The temperature of the air must be high enough to enable the drying atmosphere to hold a substantial quantity of the liquid or solvent being evaporated but should not on the other hand be so high that it will cause a chemical degradation or decomposition of the materials being dried. Moreover, it must not be so high as to melt the dried material that accumulates upon the foraminous collecting member. For example, the temperature can range up to l,000 F. In the case of food materials, the usual range is from about l00 F. to about 500F.

In some forms of the invention described hereinbelow, more than one drying atmosphere is employed. A first relatively high temperature gas is used to quickly evaporate the moisture or other solvent present in the product that is to be dried and a second or more relatively cool gas is provided for quickly removing residual heat and plasticity from the material accumulated upon the collecting screen to convert it to a relatively hard brittle and friable product. In this manner, residual surface tack if present is removed and the product is converted to a relatively stable material which can be easily handled.

The foraminous collecting member has openings which will in most instances be many times larger in diameter than the particles themselves. Thus, for example, in spray drying foods, a wire screen having sixteen to several hundred openings per square inch can be suitably employed. During the initial phases of operation, only a fraction of the spray will be deposited upon the wires of which the screen is composed. A mat will quickly begin to build up as a deposit upon the wires of the screen until the original openings within the screen are entirely closed. As this takes place, the material deposited upon the screen acts as its own collecting and supporting medium.

It is important that the surfaces of the particles are tacky at the time they strike the screen or surface of the deposited material. If they are entirely dried and exhibit no tackiness, they will either pass entirely through the screen or will tend to pack the screen and block substantially all flow of drying atmosphere through the screen which is, of course, undesirable. I have discovered that if the surface of the particles remain tacky when brought into contact with one another on the surface of the foraminous collecting screen, they will become bonded to one another in a relatively open network and will provide sufficiently large spaces between themselves to permit a satisfactory rate of flow of gas through the screen and agglomerate. On the other hand, liquid should not be present in sufficient quantity so that the particles tend to coalesce and flow from the screen or form a substantially solid mass thereon. The tack of the particles depends upon several factors including the nature of the product, the amount of initial moisture present, the pressure, temperature and vapor content of the drying atmosphere, the spray particle size, concentration and the distance of the nozzle from the collecting screen. Variation in results can be controlled by moving the nozzle toward or away from the screen.

The mount of finished material being recycled to the inlet of the apparatus will depend upon the nature of the starting material and its moisture content. Generally speaking, an increase in the moisture content of the starting material should be accompanied by an increase in the amount of finished product that is returned. Usually from 20 to 60 percent by weight of the finished product is returned to the inlet.

Refer now to FIG. I which illustrates an embodiment of the invention which can be employed for example in the drying of sucrose solutions. The apparatus of FIG. 1 comprises a vessel such as a vertically disposed chamber communicating through an inlet port 123 with a supply duct I22 and a heater 124 which in turn communicates with a dehumidifier 128 to which air is supplied by blower 130. Mounted concentrically within the upper end of chamber 120 is a nozzle 132. A sucrose solution is supplied from a material preparation storage container 133 through a line 134 to the nozzle 132 by means of a spray pump 133a. If desired, the liquid travelling through duct 134 may be aerated or any suitable gas entrained therein by means of a gas supply 135 to facilitate the drying operation and reduce the density of the finished product. The liquid sprayed from the nozzle 132 is initially present in the form of wet droplets 140. These droplets are entrained in the heated air passing downwardly from the duct 122. As the particles pass downwardly, they come in contact with dry powdered sucrose particles 142 which are introduced in an air stream through a duct 143. It was discovered during operation that the dried sucrose particles become intimately mixed with the incoming droplets of sucrose solution 140 and make it possible for the sucrose droplets 140 to be dried in crystalline form, i.e., without the formation of a glass. Moreover, while the sucrose droplets 140 formed a wet sticky syrup at the bottom of the drier, if the particles 142 were not introduced, it was discovered that the material accumulated at the bottom of the drier as a highly porous mat when the sucrose particles 142 were mixed with the spray as described. While the precise explanation is not known with certainty, it is believed due in part to the reduction in the average moisture content of the material as it approaches the bed and in part to the crystal growth facilitated by sucrose seeding.

When the falling particles reach the bottom of the chamber 120, they strike a moving foraminous element or screen 144 entrained over a pair of horizontally disposed spaced

rollers

146 and 148 suitably driven in the direction shown. The screen 144 in this case consists of articulated perforated metal sheet sections 145. A variety of other collecting elements will be apparent to those skilled in the art.

During operation, a porous mat 155 of dried sucrose forms on the upward surface of the foraminous screen 144. The air flowing into the chamber 120 from the duct 122 passes through the mat 155 as indicated by arrows into a suction plenum 162 below the screen and is exhausted through a duct 164 which is connected to a suction blower 165. The foraminous screen 144 is moved so that its upper portion thereof travels toward the right as seen in F IG. 1 carrying the accumulated mat 155 on its upward surface into a second chamber 166 through an opening 168 for purposes hereinafter described. Hot air is introduced to the chamber 166 through an inlet duct 167 which communicates with the heater 124 and is exhausted from the chamber 166 to an outlet duct 169 that is connected to an exhaust blower 170. Prior to entering the blower 130, the air is passed through a suitable filter 184. After passing through the heated drying chamber 166, the mat 155 enters a cooling chamber 171. Air is normally introduced to the cooling chamber directly from the dehumidifier 128 through a line 172. If heat is required, some air can be introduced through a line 173 from a heater 124. After passing through mat 155 and screen 144, the air is withdrawn through line 174 by blower 175. At the right end of the screen 144 is provided a doctor knife 176 for scraping any remaining material from the screen 144 before the belt returns to the inlet end of the chamber 120. The mat may be removed by any other suitable technique such as a blast of air.

The material removed from the screen 144 falls into a crusher and sizing apparatus 178 of a suitable known con struction which reduces the size of the pieces as well as controlling their density. A portion of the crushed material is then recirculated to the inlet of the apparatus in a stream of air by means of a blower 180. The particles 142 that are recirculated in this manner mix with the spray as described hereinabove.

The embodiment of FIG. 1 has a number of advantages over prior processes. For example, materials can be dried that are sensitive to moisture and tend to coalesce completely within the mat to form a liquified mass when the moisture content is raised above a critical level. Moreover, the process prevents a powder from being formed which if allowed to occur would be likely to pass through the screen 144.

In a modification of the process of FIG. 1, the ground product is not recycled. In its place a finely divided seeding substance from a separate source us used. This is accomplished by feeding a pulverulent additive material 179 composed of any of the substances described herein into hopper 181 which feeds through duct 182 to blower 183, blower and line 143 into the drier to mix with the spray from nozzle 132 in place of the material from crusher 178.

Refer now to FIG. 2 which illustrates a form of the invention used for drying relatively large food products without destroying the identity of each piece. The invention is described by way of example in the drying of fruit such as peach slices by embedding the slices in dry sucrose powder or other particu late material which is compatible with the food pieces being dried.

The apparatus is generally the same as that described hereinabove except that the recycled material is placed upon the screen 144 before it reaches the drying chamber 120. To this end, the material (sucrose) that is recycled is fed from the crushing and sizing device 178 through a duct by means of a blower 192 to the inlet end of a feed hopper 194 together with the food slices 196. The fruit slices and recycled material are deposited together to form a bed 155 composed of a mixture of the dried powder and the moist fruit slices on the portion of the screen 144 just ahead of the drying chamber 120. While in the drying chamber 120, an additional amount of particulate material initially in the form of the droplets 140 introduced through the spray nozzle [32 are intermixed with the preliminary bed 155. These particles help to make the mat more uniform in thickness and provide an extremely large surface area over which evaporation can take placev If desired, the food slices 196 can be partially dried before processing in accordance with the present invention.

A better understanding of the invention will be gained by a short description of the drying process that takes place within the bed. It should be understood that as soon as the fruit pieces are mixed with the dried particles introduced through the line 190, a certain amount of moisture will migrate to the relatively dry particles. This moisture is then distributed over the surfaces of these particles which is vastly larger than the fruit pieces themselves. As the moisture is distributed in this way, some of the sucrose particles will become dissolved but the passage of warm air through the mat within the drying chamber 120 will remove the moisture from the surrounding particles. At the same time, the relatively high concentration of solids in the moisture surrounding the fruit pieces will exert a substantial osmotic pressure in an outward direction relative to the surface of each fruit piece thereby leeching the moisture from the fruit pieces imbedded within the mat. Drying will continue in this manner with the moisture drawn from the fruit pieces by a combination of capillary action and osmotic pressure until the requisite degree of dryness is reached. The fruit pieces and sucrose particles then emerge from the drier 120 and pass into the supplemental drying chamber 166 in which most of the falling rate drying takes place. The mat then passes into the cooling chamber! 71 and finally into the product removal assembly indicated generally by the numeral 195. In the latter section the dry fruit pieces 196 are removed from the dried sucrose particles 198 by a screen 199. The dried fruit pieces then drop into a storage container 200 while the sucrose particles 198 are reduced in size by the crushing and sizing apparatus 178 and return through the line 190 as described above.

It can be seen that the invention makes possible the drying of relatively large pieces of foods such as fruits and vegetables without their being reduced to a puree or fluid form. It will also be noted that hot air is able to contact a relatively large evaporation surface composed of the multiplicity of particles surrounding each of the food pieces within the mat in which the food pieces are embedded. In this way, the moisture from the interior of each of the food pieces can be removed at a relatively high rate. Accordingly, a reduction in operating temperature exposure time is possible compared with conventional hot air drying of sliced food pieces. The invention is particularly advantageous where sugar or other solute is required in the food pieces. Where a sweet taste is not desired in the finished food piece, materials other than sugar, such as, but not limited to, dextrins, lactose or deliquescent material such as calcium chloride or sodium chloride can be used. Other materials that can be used will be apparent to those skilled in the art.

Some of the applications in which the invention can be used are shown in the following table:

F. The sucrose solution is pumped to the same nozzles used in example i at a pressure of about 1,800 p.s.i. or 80 gallons per hour. The feed rate of the liquid to the nozzle in a dry basis is EXAMPLE Ill Apple slices are dried as in example ll.

APPLICATION OF THE INVENTION TO DELIQUESCENT PRODUCTS AND COMBINATIONS OF PRODUCTS Homogeneous amorphous (non-crystalline) or Homogeneous crystalline materials (recycled or Heterogeneous amorphous or crystalline mateglassy materials (recycled or added portion acts added as desiccant only), for exampleportion acts both as desiccant and seeding mat), examples arerlals (as desiccants) in combination with fibrous particulates (food pieces), examples include Sugars, etc.: Sugars, etc.: Fr it pieces Sucros u s Vegetable pieces Lactose Lactose Megt pieces Dextrose Dextrose Legmneg uc ose uc os Artificial meats Sorbitol Sorbitol flnn o Mannitol The above foods in combination with salts, Salts Salts: sugars, etc. as desiccants Acids (citric) Sodium bicarbonate Gels and gums Sodium acetyl salicylate Acids (citric): Dextrin Cyclamate Saccharin Natural substances containing above or mixtures Natural substances Vegetable and fruit juices Whey Vegetable and fruit pulps or purees Milk Casei Jellies Jams n Carboxy-methyl cellulose elatin The term particulate material" herein includes power granule clusters as well as strands and fibers.

The invention will be better understood by reference to the following examples.

EXAMPLE l A 50 percent solids sucrose solution was dried as described in FIG. 1. The product was aerated by injecting compressed air at 300400 p.s.i. greater than the nozzle pressure of 1,800 p.s.i. (1% gal/min.) into the feed material. Powdered sucrose was introduced through the line 190 at a point about 18 inches below the spray nozzles at a downward angle of 30 with respect to the horizontal. The nozzle opening of duct 190 was about 4 inches inside the drier wall. The target air inlet temperature is 365 F. and the outlet temperature is about 200 F. Material was pumped to four No. 3 type SBC nozzles manufactured by the Spraying Systems Co. of Bellwood, Illinois The nozzles had an opening diameter of 0.026 inches. The powdered sucrose was introduced at the rate of about 3 pounds per minute. It was found that the dry sucrose mixed with the wet spray to form a finished product that dried to very fine nongranular fonn as a soft porous mat. The pressure drop across the mat and Supporting screen was about 1.5 inches of water. The bulk density of the finished product was 0.35. By comparison the bulk density of the powdered sucrose being introduced was 0.46. The particle density of the powdered sugar being introduced was 1.605 whereas in the finished product the density was L403. The moisture content of the dried material was surprisingly low; only 0.4 percent by weight. it was also found that when the flow of powdered sucrose was interrupted, the material collected as a sticky semifluid mass on the screen. The sucrose was in the glass form and as such was very difficult to dry.

EXAMPLE ii A 50 percent solids sucrose solution is dried as described in FIG. 2. A sucrose solution is aerated as in example i. Peach slices about one-half inch thick are mixed on an equal weight basis with the sucrose granules at the point at which the screen 144 enters the drying chamber 120. The air inlet temperature is held at about 365 F.; the outlet temperature at about 200 EXAMPLE lV French cut green beans are dried as in example ll except that a dextrin is employed in place of sucrose. The bed supported upon the screen 144 comprises equal parts by weight of the sliced beans and dextrin. The product is maintained within the drying chambers for a sufficient period of time to reduce the moisture content within the green beans to 5 percent by weight.

EXAMPLE v Beans are dried as in example ll except that calcium chloride is used in place of sucrose.

EXAMPLE Vl Beans are dried as in example V except that silica gel replaces calcium chloride.

EXAMPLE Vii Peas are dried as in example lV except that the dextrin is replaced with lactose. Many modifications can be made in the process. if desired, a dry edible desiccant" material can be used as an additive to the bed by blowing it into the spray chamber without the finished product being recycled. Thus, dry dextrin, especially of the low ERH type, or lactose or other compatible substance is added to a spray dried substance, e.g., coffee extract, during the process. This desiccant becomes a part of the bed and thus the final product. This process modification would be much more effective than simply adding the desiccant as a feed diluent or blending it in after the coffee was dried.

lclaim:

l. A process for drying fluids containing liquid and solid components comprising providing a foraminous collecting member, dispersing minute droplets of the fluid in a drying at mosphere, allowing the droplets to partially dry to the point where the surfaces are in a tacky condition by the time the droplets reach said foraminous collecting member, establishing a pressure drop across the foraminous member to thereby cause a portion of the drying atmosphere to pass through said member and collect said tacky droplets on the foraminous member as a porous lacy mat characterized by the ability to allow further drying atmosphere to pass therethrough, conti nuing to pass drying atmosphere through said mat until the desired degree of dryness is reached, removing said mat from said foraminous member, subdividing the mat into particulate form, returning and depositing at least a portion of the particulate material upon the foraminous collection member.

2. The process according to claim 1 wherein the returned material is reduced to powdered form.

3. The process according to claim 1 wherein the returned material is mixed with relatively large food pieces, said food pieces being deposited upon the collection member with the returned material.

4. The process according to claim 1 wherein the drying atmosphere comprises heated air.

5. The process according to claim 1 wherein said foraminous collecting member comprises an endless conveyor composed of perforated panels.

6. The process according to claim 5 wherein said conveyor is moved during the operation of said process and said mat is continuously removed from the conveyor to continuously provide a fresh collection surface for receiving and partially dried droplets.

7. The process according to claim 1 wherein a preliminary coating layer formed from the returned particulate material is applied to said foraminous collecting member, said collection member is an elongated moving conveyor element and the particulate material is placed on the conveyor before said conveyor is exposed to the drying atmosphere.

8. The process according to claim 1 wherein said mat and said screen is moved to a cooling zone and said mat is refrigerated within said cooling zone.

9. The process of claim 3 wherein the food pieces are fruit 14. The process of claim 1 wherein a dry pulverulent substance is blown into the drying atmosphere and intimately intermixed with the droplets therein and the dry finished mat is not returned and deposited on the foraminous member.

15. A process for drying fluids containing liquid and solid components comprising providing a foraminous collecting member, dispersing minute droplets of the fluid in a drying atmosphere, allowing the droplets to partially dry to the point where the surfaces are in a tacky condition by the time the droplets reach said foraminous collecting member, introducing a particulate solid material onto the foraminous member in proximity with the droplets to thereby cause said particulate material to be deposited upon the foraminous member with said droplets, establishing a pressure differential across the foraminous member to thereby draw at least a portion of the drying atmosphere through said member and collect said tacky droplets and particulate solid material on the foraminous member as a porous lacy mat characterized by the ability to allow further drying atmosphere to pass therethrough, continuing to pass drying atmosphere through said mat until the desired degree of dryness is reached.

Claims

2. The process according to claim 1 wherein the returned material is reduced to powdered form.
3. The process according to claim 1 wherein the returned material is mixed with relatively large food pieces, said food pieces being deposited upon the collection member with the returned material.
4. The process according to claim 1 wherein the drying atmosphere comprises heated air.
5. The process according to claim 1 wherein said foraminous collecting member comprises an endless conveyor composed of perforated panels.
6. The process according to claim 5 wherein said conveyor is moved during the operation of said process and said mat is continuously removed from the conveyor to continuously provide a fresh collection surface for receiving said partially dried droplets.
7. The process according to claim 1 wherein a preliminary coating layer formed from the returned particulate material is applied to said foraminous collecting member, said collection member is an elongated moving conveyor element and the particulate material is placed on the conveyor before said conveyor is exposed to the drying atmosphere.
8. The process according to claim 1 wherein said mat and said screen is moved to a cooling zone and said mat is refrigerated within said cooling zone.
9. The process of claim 3 wherein the food pieces are fruit pieces.
10. The process of claim 3 wherein the food pieces are vegetable pieces.
11. The process of claim 1 wherein the solid component is sucrose.
12. The process of claim 9 wherein the solid component comprises sucrose.
13. The process of claim 10 wherein the solid component comprises dextrin.
14. The process of claim 1 wherein a dry pulverulent substance is blown into the drying atmosphere and intimately intermixed with the droplets therein and the dry finished mat is not returned and deposited on the foraminous member.
15. A process for drying fluids containing liquid and solid components comprising providing a foraminous collecting member, dispersing minute droplets of the fluid in a drying atmosphere, allowing the droplets to partially dry to the point where the surfaces are in a tacky condition by the time the droplets reach said foraminous collecting member, introducing a particulate solid material onto the foraminous member in proximity with the droplets to thereby cause said particulate material to be deposited upon the foraminous member with said droplets, establishing a pressure differential across the foraminous member to thereby draw aT least a portion of the drying atmosphere through said member and collect said tacky droplets and particulate solid material on the foraminous member as a porous lacy mat characterized by the ability to allow further drying atmosphere to pass therethrough, continuing to pass drying atmosphere through said mat until the desired degree of dryness is reached.